1. Field of the Invention.
The invention relates to apparatus and methods for dispensing measured doses and particularly, but not exclusively, to apparatus and methods for dispensing measured liquid doses from applicators that generally comprise a plunger with a piston head sealingly fit within a cylinder. The invention is particularly applicable to applicators for veterinary purposes such as dosing animals with liquid, paste or gel medicaments. However, it will be appreciated from the following description that the invention is not limited to such applications and may be used for many other purposes where it is necessary to dispense repeated measured doses of a liquid.
2. Background.
The state of the art includes various applicator devices and methods for dispensing measured liquid doses. State of the art applicators include drenchers, trigger pumps, and syringes. These applicators or delivery systems typically comprise a cylinder and an elongate plunger. The plunger has a piston head which is slidable in a liquid-tight manner within the cylinder to discharge fluid from the cylinder through an outlet and draw fluid in through an inlet. The outlet and inlet may be the same orifice as is the case in a basic syringe. It is desirable to be able to preset the volume of liquid which is discharged from the cylinder in each compression stroke of the plunger. The preset volume of discharged liquid may be determined either by the volumetric dimensions of the cylinder as it relates to a complete plunger stroke or by a predetermined limited motion of the plunger within the cylinder. For example, there may be provided an adjustable stop arrangement for limiting the motion of the plunger either in the compression stroke direction or in the expansion stroke direction. It is preferred to limit the movement of the plunger in the expansion stroke direction to ensure that the cylinder is completely discharged after each compression stroke, regardless of the volume of liquid being discharged.
Drenchers and trigger pumps administer fixed doses of fluid either from an attached container or from an external bulk supply. An example of a trigger pump is disclosed in U.S. patent application No. 08/870,918 entitled Fluid Dispenser Apparatus filed on Jun. 6, 1997 in the name of Mark Anderson, and is hereby incorporated by reference. These devices typically have a squeezable trigger or handle used to expel the fluid from the cylinder. A fixed dose amount, predetermined by the volume of the cylinder, is accurately dispensed by moving the trigger or handle through its full range of motion, which causes the plunger to undergo complete compression and expansion stroke. Although an estimated partial dose can be delivered by partially moving the trigger or handle through its range of motion, an accurate administration of a different sized dose generally requires another applicator having a cylinder sized to contain the desired dose volume.
The syringe type devices provide a generally simpler method of dispensing doses. Syringe type devices are generally shown in the following art: Ennis, III (U.S. Pat. No. 4,923,096) which describes a dripless automatic syringe for dispensing fluids; Ennis, III (U.S. Pat. No. 5,344,409) which describes a syringe latch; Ennis, III (U.S. Pat. No. 4,852,772) which describes a dispenser for viscous fluids; Ennis, III (U.S. Pat. No. 4,678,107) which describes a dripless dispenser for liquids and viscous fluids; and Ennis, III (U.S. Pat. No. 4,981,472) which describes a cannula assembly for a syringe. Generally, a desired dose is drawn into the syringe by carefully monitoring graduated markings on the cylinder. Alternatively, a syringe latch can be attached and reattached to the plunger to repeatably and accurately meter incremental doses of fluid by limiting the compression stroke of the plunger.
The known art is believed to have significant limitations and shortcomings. Specifically, they are unable to provide an economical, efficient, and effective means for dispensing a range of accurate doses from a variety of delivery systems, including the applicators described above. Applicant's invention provides an mechanism and method for dispensing measured doses which is believed to constitute an improvement over the known art.